1. Field of the Invention
The present invention relates to a carrier for an electrophotographic two-component developer used in copying or printing machines and a developer containing the carrier. More particularly, it relates to a carrier which is particularly suitable for use in color developers, does not cause deterioration of image quality, such as color mixing, reduction of image density, edge effect, and the like, has reduced environmental dependence, and exhibits high durability and to a developer containing the carrier.
2. Description of the Related Art
It has been proposed to coat carrier particles for two-component developers with various resins so as to give a carrier improved durability against a toner-spent phenomenon.
However, a resin coat increases the resistance of a carrier, causing deterioration of image quality, specifically, reduction in image density, edge effect, and the like. The carrier resistance should be optimized by manipulating the machine system or controlling the development conditions. There are many reports on addition of a conductive substance (conducting agent) to the resin coating layer so as to adjust the carrier resistance. Various kinds of carbon black are widely known as a general-purpose conductive substance for its competitive price and ease in controlling resistance.
Addition of carbon black to the resin coating to control the resistance of a resin-coated carrier is successful in preventing image density reduction or an edge effect. However, where applied to color toners particularly of light color (e.g., yellow), carbon black is mixed into toner particles to cause contamination (color mixing).
In order to overcome the problem of color mixing which arises from use of carbon black for resistance control, the following proposals have been made to date. Japanese Patent Laid-Open No. 286429/96 discloses a double-coated carrier having two resin coating layers, the inner layer containing conductive carbon, while the outer layer containing a white conducting agent. Japanese Patent Laid-Open No. 140723/95 proposes a resin-coated carrier having a conducting agent uniformly dispersed on the surface of the carrier core but not in the resin layer. Japanese Patent Laid-Open No. 179570/96 teaches a resin-coated carrier for full color development, the resin coating layer of which contains carbon black in a concentration gradually decreasing toward the surface, falling to zero on the surface.
However, after long-term use, the coating layer of these proposed resin-coated carriers is scraped off to cause color mixing. That is, as long as carbon black is used, none of the prior arts provides a radical solution to the outstanding problems.
With regard to environmental dependence, carriers containing carbon black is heavily dependent on the environment on account of the low resistance of carbon black per se. Cases are often met with, in which charges leak particularly in a high temperature and high humidity condition, and an appreciable discharge takes place upon switching on the machine. At the beginning of operation, therefore, background staining tends to occur, and the rise of charging performance is poor, resulting in a failure to obtain a clear image.
On the other hand, a resin-coated carrier whose resin coating layer contains only inorganic oxides as a conducting agent is disclosed in Japanese Patent Laid-Open No. 35561/89, in which the resin coating layer contains at least one inorganic oxide selected from titanium oxide, zinc oxide, and tin oxide. Having high resistance per se, the inorganic oxide must be added to the resin in a much larger amount than carbon black so as to adjust the resistance at a desired level, which will reduce the durability of the resin coating layer.
With the increasing demand for high image quality, toner particles have been being reduced in size. Hence, recent studies have been directed to use in a high charge quantity area. As for a carrier core, the demands for high image quality and long-term durability have replaced high magnetic core materials such as iron powder with low magnetic core materials such as ferrite, which has higher resistance than the former. It follows that the conventional techniques have now come to be inadequate, seeing the problem that the developer has so high resistance as to reduce the image density or cause an edge effect, resulting in a failure to obtain desired image quality or a desired developer life. If a conductive substance is added in an increased amount to optimize the resistance, the coating resin will have reduced strength, which also leads to reduction of the life.